James Cooke, 38, of Ridgecrest, Calif., was in serious condition with burns to his face, hands and arms, Peterson said.

Fire investigators suspect that the workers may have been using an electric light that had not been approved as explosion-proof. Torrance Fire Chief Scott Adams said a spark from the lamp apparently touched off a brief, intense flame shortly after 11 a.m. as the two men stood on a walkway 20 feet above the ground looking through an opening on the side of a tank used in the preheating process.

Cal-OSHA is also investigating the incident but would provide no details.

Both workers are employees of a contract maintenance firm called CBI Na-Con Inc.,based in Fontana. Adams said they had been assigned to clean the unit, which had been shut down for maintenance.

Graham Harper, a spokesman for CBI's parent company in Oakbrook, Ill., said company investigators do not believe that the electric light caused the explosion.

"The light was probably not even in use, except for the night before," Harper said. "At 11 o'clock in the morning, you don't need a light. They hadn't even gone inside the (tank)."

Harper said other company employees told investigators that the explosion occurred as the workers were preparing to use a vacuum hose to clear a clogged drainpipe.

"We had three separate permits from Mobil to do the work there. All safety procedures were being followed," Harper said. "Something happened to cause an ignition, and we just don't know what that is yet."

Mobil spokesman Jim Carbonetti said both workers were wearing fire-retardant clothing and both had completed safety training the day before the accident.

"The light was there, but I don't know if it was in use," he said. "We just don't know what the ignition source was."

Although city firefighters responded to the scene, Adams said the fire had been extinguished by the time they arrived.

"We are investigating why that light was being used, who was in charge of supervision, what training had been done, that sort of thing," Adams said. "All indications are that it was not an approved, explosion-proof light."

Two workers have died and more than a dozen have been injured in a series of accidents at the refinery over the past three years. Mobil last year stepped up its safety training programs because many of the incidents involved contract workers who were not following safety procedures.